The temperature is often a critical variable that needs to be measured accurately in various industrial processes. Today, the RTD (Resistive Temperature Detector) is the most popular device used in temperature control. With an RTD element, we can obtain a resolution of hundredth or even thousandths of a degree centigrade in an ambient or moderate temperature application.
The temperature cannot be measured directly from the RTD element. It has to be calculated from the measurement of a dependant variable that has a known relation with the temperature at which it is exposed. Unfortunately this relation, which is well known by the people working in this field, is not linear and for most of the application it is not convenient to have a non-linear signal. To solve this problem, many solutions have been proposed. German Patent No. 2,459,623 to Bruyere discloses a design in which an extra resistor is connected between an amplifier output and input and a referenced resistor. This method of linearization is not that accurate, greater than one part in thousand over the entire range, and is also highly dependent on the component tolerance used in the circuit. That, obviously, poses manufacturing problems.
Other techniques have been proposed in U.S. Pat. Nos. 4,000,454 and 4,556,330 where they used an external linearization apparatus in which the voltage impressed to the conditioning section (a Wheatstone Bridge) changes as the resistance of the element changes. However, this arrangement contained also some disadvantages. First, the external placement of the linearization module provides, often, inaccurate readings by the fact that the sensing element and the linearization module are not integrated. Secondly, those devices are known to be unstable, whereby the failure of one or more components result in the failure of the entire device. Finally, the use of separate components increases the manufacturing and installation cost and can also be a problem on installation where the clearance is limited.
To overcome those problems, the U.S. Pat. No. 5,741,074 proposed a linear integrated sensing transmitter. This transmitter integrates, in a single housing, the temperature sensing device and a current transmitter. The linearization is performed via a feedback resistor, a return path resistor, and a constant current source, all connected to a voltage-to-current converter. This arrangement gives a good linearization and offers an accuracy of <0.1% of the full-scale. However, this device has also many lacks. First, the device cannot be calibrated to take in account the tolerance of the RTD element or the components themselves. That implies problems in a manufacturing point of view by the fact that we have to take a special care to the components selection. If the device is not perfectly linear, or offset, there is no way to correct it. Secondly, to change the range of operation of the device, we have to re-calculate and change all the resistor values. That is also causing a problem because we need different configuration for different range of operation.
The purpose of the present invention is to overcome those problems by suggesting an integrated transmitter, which includes the sensing element, a current transmitter and a scaler unit. The scaler unit gives more flexibility to the device and allows the calibration and re-scaling of the device via a digital communication between the device itself, and a hand-held programmer or a computer.